Self-aligned selective metal contact to source/drain diffusion

ABSTRACT

A transistor structure includes a semiconductor substrate with a first surface, a diffusion region at the first surface of the substrate, a sacrificial gate formed on the diffusion region, and insulating side walls formed adjacent to the sacrificial gate. A metal gate is formed by etching out the sacrificial gate and filling in the space between the insulating side walls with gate metals. Silicided source and drain contacts are formed over the diffusion region between the side walls of two adjacent aluminum gates. One or more oxide layers are formed over the substrate. Vias are formed in the oxide layers by plasma etching to expose the silicided source and drain contacts, which simultaneously oxidizes the aluminum gate metal. A first metal is selectively formed over the silicided contact by electroless deposition, but does not deposit on the oxidized aluminum gate.

BACKGROUND

Electroless plating is a technique conventionally used for self-aligned growth of gate metal contacts on a seed metal layer using low resistivity copper (Cu). The process succeeds primarily because Cu does not oxidize in the electroless Cu plating bath. On the other hand, aluminum (Al) readily oxidizes in aqueous plating baths so an electroless plating process has not yet been developed for a seed metal layer comprising Al. Al has a moderately higher resistivity than Cu, but a relative cost advantage. It is desirable to form self-aligned contacts on the source and drain regions while avoiding the formation of additional metallization on metal gates.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are understood by referring to the figures in the attached drawings, as provided below.

FIG. 1 shows a top view of a transistor layout according to one embodiment.

FIG. 2 shows a cross-section A-A′ of the transistor layout of FIG. 1, following a replacement metal gate process step, according to one embodiment.

FIG. 3 shows the cross-section A-A′ following an oxide etch for contact via formation according to one embodiment.

FIG. 4 shows the cross-section A-A′ following formation of a metal contact in the via by electroless plating according to one embodiment.

FIG. 5 shows the cross-section A-A′ following formation of either a carbon nano tube (CNT) or refractory metal contact layer over the electroless plated metal contact according to one embodiment.

FIG. 6 shows the cross-section A-A′ following formation of a planarized layer of deposited oxide according to one embodiment.

FIG. 7 shows the cross-section A-A′ following formation of an interlayer dielectric layer etched and filled with metallization to contact a diffusion layer and contact the gates according to one embodiment.

Features, elements, and aspects of the invention that are referenced by the same numerals in different figures represent the same, equivalent, or similar features, elements, or aspects, in accordance with one or more embodiments.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

A transistor structure having self-aligned selective metal contacts to source and drain diffusions is provided. In one embodiment, the transistor structure comprises a semiconductor substrate with a first surface, a diffusion region at the first surface of the substrate, a sacrificial gate formed on the diffusion region, and insulating side walls formed adjacent to the sacrificial gate. A metal gate is formed by etching out the sacrificial gate and filling in the space between the insulating side walls with metal. Silicided source and drain contacts are formed over the diffusion region between the side walls of two adjacent aluminum gates. One or more oxide layers are formed over the substrate. Vias are formed in the oxide layers by plasma etching to expose the silicided source and drain contacts, which simultaneously oxidizes the metal gate.

FIG. 1 shows a top view of a transistor 100 according to one implementation, where gate contacts are formed of a metal (i.e., metal gate 150), desirably by a replacement gate (RMG) process metal. In one embodiment, a sacrificial poly gate is etched out followed by a blanket metal gate fill on the wafer. The deposited metal is then polished to leave the metal in the gate region. After the polish, the top of the metal gate 150 is exposed, which may short to contacts to source and drain (S/D) unless precautions are taken to form self-aligned S/D contacts without shorting to the exposed metal gate.

In the following, one or more embodiments are disclosed, by way of example, as applicable to gate contacts formed of aluminum (Al) or copper (Cu). It is noteworthy, however, that in some embodiments any other suitable metal or compound may be utilized instead. For example, gate contact material may comprise any other element or compound that resists electroless metallization, while electroless deposition on other contacts (e.g., S/D) succeeds. Accordingly, the scope of this disclosure should not be construed as limited to Al or other exemplary material and embodiments disclosed herein.

FIG. 2 shows a cross-section A-A′ of a first intermediate stage 100 of the formation of the exemplary transistor of FIG. 1 following RMG formation of the metal gate 150. In certain embodiments, the metal gate 150 may comprise Al or Cu. For a Cu gate, a process may be implemented to form self-aligned contact using electroless plating of sacrificial cobalt (Co) deposited on the metal gate 150. For an Al gate, the electroless Co plating may not occur as the Al is readily oxidized in the plating solution. This noted property for Al, in certain embodiments, may be advantageously used to selectively deposit materials on source/drain (S/D) contacts but prevent such deposition on an Al gate.

Referring back to FIG. 2, device 100 may be formed on a substrate 110. Substrate 110 in one embodiment may comprise silicon, for example. A diffusion region 120 may be formed at the surface of substrate 110 to alter conductivity properties. In one implementation, shallow trench isolation (STI) regions 130 are formed surrounding the diffusion region 120 to electrically isolate transistor 100 from adjacent transistors or electronic device functionalities formed on substrate 110. Metal gates 150 may be formed using an RMG process. In alternate embodiments, a process other than an RMG process, such as a subtractive process, may be used to form the metal gates 150.

In accordance with one aspect, metal gate 150 is bracketed by sidewall spacers 160. Sidewall spacer 160 may be formed of an insulator such as silicon nitride, for the beneficial purpose of resisting etchants that act on oxide 180. Oxide 180 may comprise silicon dioxide in one exemplary embodiment. It is noteworthy that in certain implementations other material or combinations of different materials may be utilized to form sidewall spacer 160 and oxide 180 such that when an insulator such as oxide 180 is etched, another insulator or side wall spacer 160 is not. Diffusion region 120 may be contacted with raised S/D contacts 170 that are treated to form a silicide to enable further deposition of conductive material.

FIG. 3 shows the cross-section A-A′ of a second intermediate stage 200 of the fabrication process following an oxide etch to form contact vias 210 through oxide 180 to expose silicided raised source/drain (RSD) contacts 170. It should be noted that metal gates 150 may be exposed to subsequent processes which are provided in further detail below. Etching vias 210 in oxide 180 with a plasma oxide etch method will oxidize the metal gate 150, producing a thin insulating oxide layer.

FIG. 4 shows the cross-section A-A′ of a third intermediate fabrication following formation of a metal contact 220 in the via 210 by electroless (EL) plating, for example. EL plated metal contacts 220 may be formed of one or more of CoP, CoB, CoW, CoMo, CoWB, CoMoP, CoMoB, NiWP, NiWB, NiMoP, and NiMoB or other conductive metal alloys of Co and Ni, by way of example. The inclusion of boron (B) and phosphorous (P) in the plating bath provides electrons to assist the EL plating process. Since the metal gate 150 is oxidized during oxide etch formation of via 210 and is further oxidized in the EL plating solution, no Co may be deposited on the metal gate 150.

In some exemplary embodiments, carbon nano-tube (CNT) bundles are grown vertically on substrates with Co or Ni as the catalyst. FIG. 5 shows the cross-section A-A′ of a fourth intermediate fabrication stage 400 following formation of either a carbon nano tube (CNT) or metal contact layer 410 over the electroless plated metal contact 220 according to one embodiment.

In certain embodiments, a refractory metal (e.g., TiW) may be deposited, by a selective plasma deposition scheme, on metal contact 220. In one implementation, the refractory metal is deposited on metal contact 220, desirably, with a higher deposition rate than on oxide 180. The plasma concurrently deposits the refractory metal on the metal contact 220 while acting to remove the refractory metal on the oxide 180. The deposition and plasma etch thus form an anisotropic selective deposition of the refractory metal on metal contact 220. Since the metal gate 150 is passivated (e.g., in the form of aluminum oxide or aluminum fluoride), the refractory metal can be deposited on EL plated Co or Ni on silicide RSD contacts 170. A proper selection of plasma species, comprising oxygen or fluorine, for example, may ensure that the metal gate 150 continues to be passivated during the refractory metal deposition.

FIG. 6 shows the cross-section A-A′ of a fifth intermediate stage 500 of fabrication of the transistor following formation of a planarized layer of deposited oxide, according to one embodiment. A layer of oxide may be deposited over oxide 180 to surround (CNT) or metal contact layer 410, after which the surface may be planarized. Referring back to FIG. 1, gate contact 10 can be patterned and formed (e.g., adjacent to the diffusion region 120) at this point. A sequence of various processes, including photomasking, oxide etching, and metal deposition may be employed to form gate contact 10. In one embodiment, CNT layer 410 may be etched out and replaced by the same metal contacting the gates during the deposition process for forming the gate contacts 10.

FIG. 7 shows the cross-section A-A′ following formation of an oxide 184 (e.g., interlayer dielectric layer (ILD)) over oxide 182, which may be etched and filled with final contact metallization 190 to contact the diffusion layer 120 by way of contact layer 410, metal contacts 220 and silicided source and drain contacts 170. The same oxide 184 may be, for example, patterned and etched to enable formation of contacts to the gates 150 by etching a region of oxide layers including one or more of 180, 182 and 140 opposite diffusion layer 120, as shown in FIG. 1. The contact metal 190 in one embodiment may be comprised of copper or tungsten.

For simplification, it is appreciated that the methods and structures disclosed herein may not require replacement of a metal by an insulator to protect the gate, as the gate oxide layer that forms on Al may be sufficient to prevent electroless deposition thereon while disposing metal on other regions containing catalyst atoms to support deposition, such as, for example, electroless plating.

The various embodiments described above have been presented by way of example and not by way of limitation. It should be understood that the processes, methods, and the order in which the respective elements of each method are performed are purely exemplary. Depending on the implementation, they may be performed in a different order or in parallel, unless indicated otherwise in the present disclosure.

The method as described above may be used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multi-chip package (such as a ceramic carrier that has either or both surface interconnections of buried interconnections).

In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

Having thus described in detail embodiments of the present invention, it is understood that the invention defined by the appended claims is not to be limited by particular details set forth in the above description, as many apparent variations thereof are possible without departing from the spirit or scope thereof. 

1. A method of forming a transistor structure comprising: providing a semiconductor substrate having a first surface; providing a diffusion region at the first surface of the semiconductor substrate; forming a sacrificial gate over the diffusion region; forming insulating side walls adjacent to the sacrificial gate; forming silicided source and drain contacts over the diffusion region adjacent to the side walls; forming a first oxide layer over the substrate; forming an aluminum gate by etch removal of the sacrificial gate and filling a space formed between the insulating side walls with aluminum; applying a plasma oxide etch to form at least one via in the first oxide layer to expose the silicided source and drain contacts; and selectively depositing a first metal layer in the via.
 2. The method of claim 1, wherein the side walls are insulators with etch properties that are selectively different from the first oxide layer and the sacrificial gate.
 3. The method of claim 2, wherein the aluminum gate is formed by a replacement metal gate process.
 4. The method of claim 2, wherein the plasma oxide etch produces an oxide or fluoride on the aluminum gate.
 5. The method of claim 4, wherein the first metal layer is selectively deposited on the silicided raised source and drain contacts using electroless plating.
 6. The method of claim 5, wherein the first metal layer comprises at least one of CoP, CoB, CoW, CoMo, CoWB, CoMoP, CoMoB, NiWP, NiWB, NiMoP, NiMoB, or a combination thereof.
 7. The method of claim 5, wherein the oxide or fluoride formed on the aluminum gate prevents electroless plating of the first metal layer on the aluminum gate.
 8. The method of claim 7, further comprising selectively depositing a second metal layer over the first metal layer.
 9. The method of claim 8, wherein the second metal layer comprises vertically grown carbon nano tube bundles.
 10. The method of claim 8, wherein the second metal layer comprises a refractory metal.
 11. The method of claim 10, wherein the second metal layer is deposited by plasma deposition, wherein the plasma substantially inhibits formation of the second metal layer on oxides or fluorides, including the oxided or fluorided aluminum gate.
 12. The method of claim 8, further comprising: depositing a second oxide layer over the second metal layer; and planarizing the second oxide layer to expose the second metal layer.
 13. The method of claim 12, further comprising: forming a second via in the second oxide layer to gain access to the aluminum gate; and filling the second via with metal to form an interconnect to the aluminum gate.
 14. The method of claim 13, further comprising: forming a third via in the second oxide layer to gain access to the second metal layer; and filling the third via with metal to form an interconnect to the second metal layer.
 15. The method of claim 12, wherein the second oxide layer comprises vertically grown carbon nano tube bundles, the method further comprising: removing the vertically grown carbon nano tube bundles from the second oxide layer, wherein the removing forms a hole in the second oxide layer providing access to the first metal layer; and depositing a metal in the hole in the second oxide layer to form a contact to the first metal layer.
 16. A transistor structure comprising: a semiconductor substrate having a first surface; a diffusion region formed at the first surface; an aluminum gate formed over the diffusion region; an oxide or fluoride layer on the aluminum gate; insulating side walls formed adjacent to the aluminum gate; silicided source and drain contacts formed over the diffusion region adjacent to the side walls; a first oxide layer formed over the substrate; one or more vias formed in the first oxide layer over the silicided source and drain contacts; and a first metal layer selectively deposited in the via to form a metal contact.
 17. The structure of claim 16, wherein the side walls are insulators with etch properties that are selectively different from the first oxide layer and the sacrificial gate.
 18. The structure of claim 17, wherein the oxide or fluoride layer is formed by a plasma oxide etch that is used to form the vias in the first oxide layer.
 19. The method of claim 18, further comprising a second metal layer deposited by a plasma deposition process that substantially inhibits formation of the second metal layer on the oxide or fluoride layer.
 20. The method of claim 19, wherein the second metal layer comprises vertically grown carbon nano tube bundles.
 21. The method of claim 19, wherein the second metal layer comprises a refractory metal. 